Radiographic imaging of the body is well known and extremely useful as a diagnostic tool in the medical arts. Radiographic imaging involves positioning a part of a patient to be imaged denoted as the "structure of interest" under an X-ray tube, exposing the structure of interest to an X-ray beam, and recording the X-ray image on an image receptor. The receptor in most instances is a radiographic film disposed in contact with an intensifying screen. The intensifying screen absorbs x-ray radiation and radiates light in proportion to the radiation absorbed. Light emitted by the intensifying screen exposes the film. The film and screen are kept in tight contact during exposure in a film holder or cassette. After exposing the structure of interest, the film is removed from the cassette, may be labeled with the patient's name and other identifying information, and then developed. The use of radiography to image the-human female breast is referred to as mammography.
Mammography is today the most important and accurate method for diagnosing breast disease. The diagnostic value of mammograms is highly dependent on image quality which, in turn, depends upon the interplay of several factors: the size, the angle, and elemental composition of the x-ray target, the energy spectrum of the x-ray beam, the type of imaging system, the image processing system, patient radiation dose, etc. Recent studies by the inventor and by others have shown that there is a wide variation in the quality of mammograms being produced at breast imaging facilities in the United States.
Because of the importance of good image quality in mammography a number of regulatory agencies and professional groups have recommended that mammography facilities have an ongoing quality assurance program. One of these professional groups initiated a voluntary accreditation program for mammography centers in 1987. In order to become accredited a mammography facility must, among other things, submit a radiographic image of a prescribed phantom which contains artifacts that simulate breast calcifications, tumors and fibers. Image quality is determined subjectively, i.e., by visual assessment of a known phantom image having a number of known artifacts.
It is well known in the field of radiology to utilize an inanimate object in place of a patient in conducting serial x-ray exposures for calibration or similar purposes. Such an object is called a phantom and in mammography the object is called a breast phantom. Breast phantoms are composed of materials, for example, certain plastics and wax, that simulate the x-ray absorptive characteristics of a human female breast. Small discrete objects are incorporated in a phantom to differentially absorb x-rays in a manner similar to that encountered in a clinical situation in order to produce an image on a receptor such an x-ray film. These objects are usually called artifacts. In some phantoms the artifacts are imbedded in a wax plate that fits into the plastic body of the phantom. In others the artifacts are imbedded in the plastic itself. The artifacts are also sometimes configured to simulate the physiological shape and approximate size of important clinical markers such as tumors, fibrils and calcifications. State of the art phantoms usually incorporate nylon fibers of different diameter to simulate fibrils, different size particles of alumina oxide, calcium carbonate, calcium hydroxyappotite, calcium sulfate, etc. to simulate breast calcifications and cross sections of nylon spheres to simulate tumors. These artifacts are positioned in the phantom in an arbitrary but generally reproducible way and nominal specifications regarding size and sometimes thickness are provided by the vendor. The phantom used in the accreditation program consists of a plastic block and a wax insert that contains the artifacts. A radiograph of the insert by itself, that is without plastic block, is provided to the user to demonstrate the location of the artifacts. This image is also intended to demonstrate the maximum number of artifacts that can be visualized in a contact radiograph with essentially no excess scatter. In the accreditation program, the complete breast phantom is radiographed and the scatter helps to reduce the number of artifacts seen. Viewers score image quality on the basis of the number of artifacts seen.
The radiation dose and beam quality used to produce the phantom mammogram is calculated from phantom surface exposure measurements, using solid state detectors, i.e., thermoluminscent dosimetry (TLD). These calculations are typically made by a commercial firm independent of the mammography centers or the accrediting organization, the American College of Radiology.
The diagnostic value of radiographic imaging as described above is dependent on the quality of the radiographic image, which in turn depends on an interplay of several factors. One of the more important of these factors is the process by which the radiographic image is developed. Radiographic images which are made on radiographic films are generally developed in devices called "film processors." Film processors are subject to many variations which are functions of the kind of film processor used to develop the film, the age and quality of the chemicals in the film processor which develop the film, the duration of time the film is processed, and the temperature and pH of the chemicals. Since the diagnostic value of a radiographic image is highly dependent upon the quality of the radiographic image, it is imperative that the film processor be well controlled in order to optimally develop the image. It is also important to minimize fluctuation of processor parameters from film to film.
Processor performance, determined by sensitometry/densitometry measurements of the processor used to develop the radiograph, is generally made in-house by the facility seeking accreditation. A separate thirty day record of processor performance is submitted as part of the documentation for accreditation. However, none of these records are directly linked to the phantom image so that the effect of processor performance on the phantom radiograph cannot taken into account in evaluating image quality.
Data from several thousand mammography facilities participating in this program have now been collected. These data continue to show a wide variation in patient dose and image quality, even amongst facilities that are accredited in the aforementioned fashion.
The same professional group has recently introduced a program to re-accredit mammography centers once every three years and to update accreditation on a more frequent basis. The method used for image evaluation and the criteria for re-accreditation remains essentially the same.
The inventor of the subject matter herein claimed and disclosed has recognized a need in the art to improve the current method used to evaluate image quality in mammography. Specifically, a need exists for objective image quality standards rather than the subjective type evaluation now being used in the accreditation program. Such quality standards should be based on physical measurements rather than subjective impressions.
Moreover, processor performance is a critical parameter that needs to be incorporated in any determination of mammographic image quality and the establishment of image quality standards.
A need exists for more frequent independent monitoring of image quality in mammography once in three years, and that this monitoring too, should incorporate a measurement of the effect of film processing on image quality.
As is known by those with skill in the art, an X-ray image of acceptable diagnostic quality generally comprises an image of the structure of interest as a series of gray levels. Examination of the gray level image indicates whether the structure of interest is healthy, or whether the structure of interest may contain certain diseases such as, for example, cancer.
Since the quality of the radiographic image is highly dependent on the film processor, the film processor must be periodically tested to ensure that the images which are produced have a high diagnostic quality. There are several prior methods currently in use to test film processors. One such method involves the use of a "sensitometer" and a "densitometer." A sensitometer is an instrument which impresses a series of graduated exposures on a photographic material. In these sensitometers, a light source of known luminous intensity is displaced at a fixed distance from an exposure plane and emits radiation of known spectral intensity. The surface of the photographic material is positioned to substantially coincide with the exposure plane.
In the sensitometer, an exposure modulating device is located between a film and the light source. If the exposure modulating device is removed, the entire photosensitive material may be uniformly illuminated. However, the purpose of the exposure modulating device is to alter this condition so that various areas of the photosensitive surface are subjected to a series of different exposures, thereby forming a graded density pattern on the photosensitive surface which is developed as a series of gray levels. This density pattern is a function of the type of film and the action of the processor.
After the film is developed by a film processor with the sensitometric graded density gray scale level pattern imposed thereon, a densitometer is used to measure densities created by the exposure modulated device. In this fashion, the graded density pattern, which may be precalibrated in terms of various parameters such as for example, film speed, base and fog, and contrast, can be used to gauge and evaluate the performance of the film processor.
Various other methods and apparatus have been used to test film processors. Examples of these other methods and apparatus are sensitometric film strips which have been pre-exposed and aged, and are then packaged to be sold commercially. These pre-exposed strips are used in conjunction with a readout device. To check the film processor, one of the strips is developed and inserted into the readout device. When the film is withdrawn, the readout device produces a light signal which indicates the temperature and the condition of the chemicals in the processor. No digital readout is provided and no quantitative indications of the condition of the film processor can be determined.
Methods to check film processors by measuring the pH of the chemicals and the operating temperature of the film processor are also known in the art. It has also been known to use "step wedges" to create a graded pattern on radiographic films. These step wedges are generally constructed of an X-ray absorbing material and are used to determine the effect that the X-rays have on the image quality, but not the effect that the film processor has on the image quality.
The aforementioned prior methods for testing a film processor which develops radiographic images do not satisfy long-felt needs in the art for methods and apparatus to test film processors that are quick, efficient and standardized to particular exposures and film types. The recommended frequency for conducting sensitometric and densitometric tests is daily. However, in the realities of the clinical environment, daily testing of film processors is often not completed.
There are many reasons that daily testing is not always accomplished. Chief among these reasons are that special training and equipment are needed, and additional X-ray film is required. As a result, the diagnostic quality of X-ray images is often severely compromised. Poor film processor performance results in degraded radiographic image quality and could ultimately result in failure to detect diseases. This is particularly devastating, for example, in radiographic images of female breasts called "mammograms" where diagnostic features are often subtle, and early detection of breast cancer is often critical to future survival.
The inventor of the subject matter herein claimed and disclosed has recognized a long-felt need in the art to eliminate repeat densitometric readings of test films to monitor the performance of film processors. There are further long-felt needs in the art to minimize the use of extra test films to monitor processor performance, and also to provide the ability to record the effect of processing on the radiographic image for recall during subsequent examinations. A permanent record of processor monitoring for quality assurance and medico-legal needs is also desired in the art.
In sum, breast phantom image quality depends on the interplay of several factors--kVp, mAs, HVL, film/screen system, processing, etc. Current quality control and map test methods are not able to measure the effect on image quality due to changes in one or more of these parameters because of: 1) the limited amount of information that can be elicited from an image of only a breast phantom; and 2) the subjective (visual) method used to assess image quality.